1. Field of the Invention
This invention relates generally to manufacturing filters for inflatable type modular occupant restraint systems for passenger vehicles or, as they are commonly known, air bag restraint systems. The filter in such systems functions to cool hot gases before they reach the air bag and serves to trap particulates and residues generated during ignition so that they do not enter the air bag and escape into the vehicle. More particularly, this invention relates to an improved method for manufacturing inflator filters which includes re-sizing and re-shaping said filters by subjecting the filters to a rotary swaging operation.
2. Description of The Related Art
Filter assemblies generally available in the art include alternating layers of screen and ceramic paper. Paxton et al, U.S. Pat. No. 4,998,751, which is assigned to the assignee of the present invention, disclose a filter assembly which comprises two wraps of nickel coated carbon or stainless steel 30-mesh screen, metal filters such as 80.times.700 or 50.times.250-mesh stainless steel or 40.times.180-mesh nickel coated carbon steel, a single wrap of ceramic filter paper 0.080 inches thick followed by two wraps of 30-mesh stainless steel or nickel coated carbon steel.
The filter assemblies are, typically, manufactured by wrapping the different filter materials circumferentially around a mandrel. Cunningham, U.S. Pat. No. 4,878,690, which patent is assigned to the assignee of the present invention, discloses a cylindrical filter assembly manufactured accordingly. The filter assemblies are manufactured as single units by hand and/or machine. The particular geometry of the filter assemblies and the number of wraps of filter materials are determined by the designed end use of the assemblies.
There is a roundness problem associated with making cylindrical filter assemblies for air bag inflators, which have wrapped multiple layered or stack-up structural configurations. The use of such wrapped multiple layers or stack-ups inherently results in a screen pack assembly exhibiting an out-of-round, i.e., ellipsoidal, cross-section. This inherent lack of roundness is exacerbated as the number of layers of filter materials increase and is particularly acute at the overlap outside diameter (O.D.).
Re-sizing cylindrical filters for use in air bag inflator systems has been a topic of concern in the art dating back to early driver side air bag applications. However, on passenger side air bag programs resizing of oversize filters was a dead issue from the beginning since the diameter requirement was only a maximum. More recently, the implementation of a minimum diameter requirement coupled with existing maximum requirements has revived interest in re-sizing. One re-sizing method comprises reworking oversize passenger screens by sizing them down to size through the use of a split tubular (clam shell) stationary die mounted to a press. Use of the clam shell method for re-sizing has not proved satisfactory because it requires operating with a minimal oversize tolerance in order to prevent screen damage. With a clam shell die the majority of the working occurs at the die separation region and this results in localized working. There is also a finning or winging problem associated with the use of a clam shell die when large diameter as-wrapped screen packs are worked. Thus, the use of a clam shell die places restrictions on the as-wrapped O.D.s of the screen pack that can be successfully re-sized. Further, the inherent ellipsoidal shape of the as-wrapped filter screen packs requires special placement of the screen pack into the clam shell die. The maximum diameter of the ellipsoidal shaped screen must be aligned with the vertical axis of the die member to effect re-sizing and re-shaping of the as-wrapped screen pack.
The consistent and economical manufacture of filter assemblies for air bag inflator systems, which efficiently cool and clean gas from the gas generant, is of prime concern. The implementation of a resizing process for producing cylindrical filter assemblies exhibiting superior dimensional characteristics such as diameter, wall thickness, roundness, straightness, cylindricity, etc. and which exhibit lower residue levels has obvious advantages and benefits.